One problem in feeding a top or bottom sheet of media from a stack of sheets of media is that the sheets may stick together and at least the next adjacent sheet may be fed at the same time. Accordingly, various separating means have been suggested for separating a top sheet of a stack of sheets of media from the next adjacent sheet when the feed is from the top of the stack of sheets of media and for separating a bottom sheet of a stack of sheets of media from the next adjacent sheet when the feed is from the bottom of the stack of sheets of media. This problem is particularly acute when feeding labels, as labels are formed on a thick medium with numerous ridges and valleys that interlock from one sheet to the next, causing the sheets to stick together. Also, certain label materials, such as vinyl, tend to stick together.
It is known to separate a top sheet of a stack of sheets from the next adjacent sheet through using a dam, which is an element having an inclined surface in the path of the top sheet, as it is fed from the stack of sheets, so that its leading edge will strike the inclined surface of the element. In a printer, however, the advancement of more than one sheet from the stack of sheets can cause jamming. Therefore, it is necessary to avoid simultaneous advancement of more than one sheet from a stack of sheets of media to a processing station such as a printer, for example.
Additionally, a surface having a coefficient of friction capable of reliably separating heavy media such as labels, for example, tends to fail to feed at all sheets of a light weight or high friction media such as bond or xerographic paper, for example. This presents the problem of whether an inclined surface of a dam should be capable of reliably feeding and separating heavy media or light media. This is not desirable with a printer since a printer needs to be capable of printing both heavy and light media to have a sufficient market.
U.S. Pat. No. 5,899,450 to Gettelfinger et al. discloses a sheet separator dam with a plurality of ribs and an inset friction element, such as a pad. The pad surface has a coefficient of friction substantially equal to the sheet to sheet friction. If more than one sheet is fed from a stack to the dam, the sheets strike the rib surfaces of the dam first, and then the pad surface, if not separated from each other by the rib surfaces. The frictional step function separates the top sheet from the next adjacent sheet. In such an arrangement, however, the pad must be made of a material having a high coefficient of friction, such as rubber or a special plastic, which is relatively expensive. The pad wears in time and must be replaced at substantial expense. Also, a rubber pad may mar bond paper and leave an undesirable black mark. Still further, this arrangement does not reliably separate the top sheet where the sheets have unusual characteristics, such as labels.